Petroleum refiners have been removing salts from crude oil and product fractions almost since the dawn of refining.
Crude oil contains salts and salt precursors such as nitrogen and sulfur compounds. Generally, this salt consists of a mixture of sodium chloride, magnesium chloride, and calcium chloride. Salt is removed upstream of fractionation equipment to prevent corrosion and plugging in columns and associated equipment. Simple water washing will remove most of these chloride salts, and most refiners now use "desalters."
Modern refining techniques also produce salts or sometimes introduce salts into process streams.
The main source of the produced ammonium salts is chemical reactions between an acidic or basic compound dissolved in the oil and an acid or base added or created during refinery processing. Thus nitrogen and sulfur compounds in the feed are frequently converted to hydrogen sulfide or ammonia. Another impurity, hydrogen chloride, can be produced by hydrolysis of calcium and magnesium chlorides or by hydrogenation of organic chlorides. Ammonia will react with hydrogen sulfide and hydrogen chloride to produce ammonium hydrosulfide and/or ammonium chloride, respectively.
Salts can be introduced into a refinery stream when a catalyst contains leachable alkaline or acidic components. Thus platinum reforming catalyst can introduce chlorine into reformate and to the reformer off gas.
These salts, regardless of source, can form either solid deposits or an aqueous phase if water is present.
Although salt deposits, or formation of salt solutions, might seem like minor problems the opposite is the case. Salt deposits can plug distillation column internals, and shut down an entire refinery. The salts are also hygroscopic and attract water to form extremely corrosive regions. Corrosive salt solutions can and do eat holes in refinery vessels.
As an example of how serious the problem can be, on May 5, 1988 at about 3:30 a.m. an explosion occurred at a Louisiana refinery which resulted in 7 fatalities, 28 injuries, and significant property damage.
A depropanizer associated with a large FCC unit had experienced condenser fouling. To control fouling (salt buildup in the condenser tubes) water washing was begun. Wash water containing ammonia was injected into the vapor stream to maintain the pH in the condenser boot between 6.5 and 8. Despite use of an alkaline water wash, a localized region of high H.sub.2 S absorption and H.sub.2 S acidification developed downstream of the water injection nozzle. The resulting iron sulfide corrosion products were a porous scale at this point in the pipe, which allowed more H.sub.2 S to accumulate. The net result was that about 100 square inches of metal was ejected from an 8" pipe elbow about 1.1 m downstream of the water injection nozzle. This depressured the depropanizer into the refinery, forming a vapor cloud which exploded. More details of the failure are reported in the paper entitled "A new form of localized corrosion" in Materials Performance Vol. 32, No 6, June 1993, which is incorporated by reference.
It is also worth noting that although the explosion occurred in May of 1988, it took roughly five years, until June of 1993, to publish the report, which refers to "unpredictable high rates of H.sub.2 S absorption at the turbulent periphery of the water splash zone . . . . "
This paper and the disaster exemplify the state of the art in refinery processing, namely controlling water injection based on boot water pH, and relying on inspection to check for localized corrosion problems. Such a retrospective approach can lead to tragic results, though it may be that nothing could have prevented the localized corrosion which occurred in that FCC unit.
We discovered that much of the corrosion that occurs in refinery and chemical process equipment is predictable and avoidable. Before getting into our new method of controlling water washing, it is instructive to review salt formation or introduction and conventional practices associated with salt removal. The review is by no means complete, but illustrates some of the complexities of salt formation and removal. Salt formation in two "wet" processes (crude distillation and hydrotreating) and one "dry" process (catalytic reforming) will be considered.